Pour the stout and white wine vinegar into a medium saucepan or fondue pot. Bring to the boil over medium-high heat, then reduce to a gentle simmer.
While the liquid is heating, grate the cheddar cheese if you have not already. Place the grated cheese in a large bowl and toss thoroughly with the cornflour until all the cheese is lightly coated.
Once the stout is simmering, reduce the heat to low. Add the cheese in small handfuls, stirring constantly and waiting for each addition to melt completely before adding the next. This should take about 5-7 minutes total.
As you add the cheese, stir in the crushed garlic, Dijon mustard, Worcestershire sauce, nutmeg and black pepper.
Continue stirring over low heat for another 2-3 minutes until the fondue is completely smooth and velvety. Do not let it boil.
If using a fondue pot, transfer the cheese mixture to the pot and place over a small burner on the lowest heat setting. If you do not have fondue equipment, you can serve it directly from the saucepan and reheat gently as needed.
Serve immediately with cubes of crusty bread and apple slices for dipping.